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Cassini continues its exploration of the Saturn system with the 28-day Rev 236, which begins on May 20 at its farthest distance from the planet. This is also called the orbit’s apoapse. At this point, Cassini is 3.21 million kilometers (2.00 million miles) from Saturn’s cloud tops. Rev 236 occurs during the second inclined phase of the Cassini Solstice Mission. Over the next several orbits, Cassini will use encounters with Titan to gradually increase the inclination of its orbit. Forty-two ISS observations are planned for Rev 236 with the majority focused on Saturn’s atmosphere, its system of rings, and Titan.
For Cassini’s first observation of Rev 236, on May 21, ISS will look at Titan’s northern trailing hemisphere from a distance of 3.69 million kilometers (2.29 million miles). ISS will be looking for clouds across Titan’s north polar region and at northern mid-latitudes during this sequence. Five similar cloud monitoring observations will be acquired between May 25 and June 4. The closest of these will be the second cloud monitoring observation on June 4, acquired from a distance of 1.50 million kilometers (0.93 million miles) and will be focused on Senkyo and eastern Aztlan. Immediately after the Titan cloud observation on May 21, ISS will acquire an astrometric observation of Saturn’s small, inner moons. Astrometric observations are used to improve our understanding of the orbits of these small satellites, which can be influenced by Saturn’s larger icy moons as well as each other. Careful measurements of the positions of these moons are important for later imaging of them at much closer distances during the F ring orbits later this year and early next year. Four more astrometric observations will be acquired between May 25 and June 1. Another will be acquired on June 9.
After the astrometric observation on May 21, ISS will acquire a quick observation of Saturn using the Wide-Angle Camera (WAC). This observation is part of a series of “Storm Watch” sequences designed to take advantage of short, two-minute segments when the spacecraft turns to point the optical remote sensing (ORS) instruments back at Saturn, as a waypoint between observations. They include blue, clear, two methane band, and one full-frame, continuum band filter images. Nine more Storm Watch observations will be taken between May 22 and June 2. Another two will be taken on June 10 and June 11.
On May 25 and 26, ISS will observe the distant moon Albiorix for 28 hours, covering roughly two Albiorixian days. The observation will be taken from a distance of 19.8 million kilometers (12.3 million miles). Between June 11 and June 17, ISS will acquire three similar observations of Tarqeq covering nearly 90 hours. While Albiorix and Tarqeq will only appear as point of lights, these two observations can be used to measure the length of their days as well the orientation of their spin axes. On May 30, ISS will track the G ring arc for 14 hours.
On June 5 at 11:45 UTC, Cassini will reach periapse for Rev 236 at an altitude of 545,330 kilometers (338,850 miles) above Saturn's cloud tops, outside the orbit of Rhea. In addition to a pair of Titan cloud monitoring observations, ISS will acquire a pair of ride-along observations of Saturn’s icy satellites during the periapse period. On June 3, ISS will ride along with the Ultraviolet Imaging Spectrometer (UVIS) to observe Dione at very low phase angles from a distance of 950,000 kilometers (590,000 miles). Later that day, ISS will ride along with the Composite Infrared Spectrometer (CIRS) to observe Rhea’s anti-Saturn hemisphere from a distance of 582,000 kilometers (362,000 miles). Also during periapse, UVIS will acquire a pair of Saturnian south polar aurora observations on June 4 and June 5. On June 6, the Radio Sub-System (RSS) will perform a radio occultation of Saturn’s main ring system.
On June 7 at 14:06 UTC, Cassini will perform a targeted encounter of Titan. This is Cassini’s 121st flyby of Titan, the sixth of eleven planned for 2016. The next encounter is planned for July 25 during Rev 238. T120 has a close approach flyby altitude of 975 kilometers (606 miles). This encounter will increase the inclination of Cassini’s orbit from 35.3 degrees to 42.4 degrees and will decrease the spacecraft’s orbital period from nearly a month to 24 days. On approach, the Composite Infrared Spectrometer (CIRS) will view the sub-Saturn hemisphere of Titan, acquiring nadir and limb sounding observations at mid- and far-infrared wavelengths. Many of these are designed to measure seasonal changes in atmospheric temperatures and trace gas abundances, particularly near the edge of Titan’s south polar vortex. CIRS will also measure how haze opacity varies with altitude. ISS will ride along with these observations.
At closest approach, RADAR will acquire altimetry and SAR swaths across portions of Titan’s southern and anti-Saturn hemispheres. The SAR imaging swath will run from near 55 degrees South Latitude, 30 degrees West Longitude (northwest of Mezzoramia) west-southwest to 65 degrees South Latitude, 90 degrees West Longitude, before turning northwest to cover portions of Hobal and Perkunas Virgae and Shangri-La before ending near Texel Facula. This is the last RADAR SAR swath to cover high-southern latitudes on Titan. Outbound, CIRS will perform additional atmospheric compositional measurements. Late of June 8, ISS will acquire a series of cloud tracking observations covering Titan’s north polar region.
On June 10, ISS will acquire a haze monitoring observation of Titan at a distance of 1.38 million kilometers (0.85 million miles). On June 10, ISS will take a movie of the F ring, lasting 15 hours. On June 11, ISS will acquire a color mosaic of Saturn’s rings using the Narrow-Angle Camera and a color mosaic of the entire main ring system and Saturn using the Wide-Angle Camera as a sort of parting shot for Cassini.
On June 17, Cassini will reach apoapse, bringing Rev 236 to a close and starting up the next orbit, Rev 237.
Image products created in Celestia. Rhea basemap by Steve Albers. All dates in Coordinated Universal Time (UTC).